Question about sustained winds

[docjoe]

Could someone elaborate on the whole "sustained wind" system. Why does it take a 60 second measurement to be a sustained wind? Is this an average of wind speed over a given minute? Would 50 seconds of 115MPH wind with 10 seconds of 75 MPH NOT be considered cat 3?? I am getting confused here

docjoe

[brunota2003]

It has to do with "Maximum Sustained Winds", but it should help you I hope: http://www.aoml.noaa.gov/hrd/tcfaq/D4.html

[TS Zack]

It is just a minute of wind averaged out.

You will have winds of 60mph, then 115mph, then 76mph, then a huge gust to 140mph. Equating to a sustained wind of 97mph. You will never have the same wind for a minute. I was talking to a pro met that rode out Katrina in Covington and he said it would just be windy then it would just come blowing through real hard for 30 seconds, then let up. Stay windy and do it all over again. He recorded wind gusts to 120mph, so the gust are a whole lot stronger than the sustained winds.

[docjoe]

It is just a minute of wind averaged out.

You will have winds of 60mph, then 115mph, then 76mph, then a huge gust to 140mph. Equating to a sustained wind of 97mph. You will never have the same wind for a minute. I was talking to a pro met that rode out Katrina in Covington and he said it would just be windy then it would just come blowing through real hard for 30 seconds, then let up. Stay windy and do it all over again. He recorded wind gusts to 120mph, so the gust are a whole lot stronger than the sustained winds.

but why was a minute picked? Seems to me that wind of any speed that last for 20 or 30 seconds is sustaining. You certainly dont need 60 full seconds of 110+ MPH to do a whole lot of damage. I wonder if an overall better understanding of this would help some of the threads under other topics recently?

docjoe

[P.K.]

Due to the turbulent nature of wind the shorter the measurement period the more it is affected by short time scale changes and so not be representative of the wind in that area. Now if you measure over a longer period (Say 10 minutes which I favour) then you filter out some of this short scale variablity and get a more reasonable figure for that period. That said I quite like the Australian TC scale which uses only gusts.

[Aslkahuna]

Well 10 minute winds are the standard set by WMO for sustained winds. I think we are about the only major country that uses one minute-at least we use the standard 10m height. I suspect that we use one minute because our Met reporting criteria are coordinated between the various agencies that use and report Met data such as the FAA, USAF, USN, NOAA, etc. whereas elsewhere, everyone is told by the official Government Agency what they will use and it wll be the WMO standard.

Steve

[caneflyer]

A web search reveals the following from an article by Franklin in Natural Hazards Review. Basically, the one-minute averaging is a holdover from the old fastest mile concept:

The use of the highest 1-minute average wind to define a hurricane therefore is the NWS standard, not simply an NHC standard as Sparks asserts. The earliest published use by the NHC in the formal literature was in the season summary article describing the 1980 hurricane season (Lawrence and Pelissier 1981). The term "sustained wind" appears in earlier NHC articles, but prior to 1980 (e.g., Simpson and Hope 1972) the term appears to be have been used interchangeably with the "fastest mile wind" (the reciprocal of the shortest interval that it takes one mile of air to pass a given point). The fastest mile was commonly reported by land-based stations with multiple-register anemometers, especially prior to 1980. As technology changed, it became customary to report winds averaged over various periods of time, rather than over distances, and the "fastest mile" is no longer reported by U.S. observing stations. As the sustained wind concept evolved from a distance-based mean to a time-mean, a one-minute averaging period was chosen by the NHC because it represents averaging similar to the fastest mile for tropical cyclone wind speeds. (The equivalence, of course, is exact at 60 mph. The fastest mile speed equivalent for Sparks' preferred standard of a 10-min average would be 6 mph, a speed of little interest to NHC).